Change 'YOUR_APP_SECRET' to the API secret from the Facebook page in step 10.

Save the configuration file.

Reconfigure or restart GitLab for the changes to take effect if you
installed GitLab via Omnibus or from source respectively.

On the sign in page there should now be a Facebook icon below the regular sign in form. Click the icon to begin the authentication process. Facebook will ask the user to sign in and authorize the GitLab application. If everything goes well the user will be returned to GitLab and will be signed in.